1. Field of the Invention
The invention relates to a method of production of a methionine salt, in particular the production of a methionine salt starting from the precursors 3-methylmercaptopropionaldehyde (MMP) and hydrogen cyanide (HCN) or starting from at least one component that can be prepared from these raw materials, such as methylmercaptopropionaldehyde-cyanohydrin (MMP-CN). In particular the invention relates to the alkaline hydrolysis of 5-(2-methylmercapto)-hydantoin in a column. The invention further relates to a reaction system suitable for this method, comprising a reactive-rectification column, and the use of the reaction system.
2. Discussion of the Background
The industrial synthesis of racemic methionine (mixture of 50% L-methionine and 50% D-methionine) starts from petrochemical raw materials, in particular propene, sulphur, methane and ammonia. According to usual methods, the precursor 3-methylmercaptopropionaldehyde is thus prepared via the precursors acrolein, methylmercaptan and hydrogen cyanide. Then this aldehyde is converted with ammonia, carbon dioxide and hydrogen cyanide to 5-(2-methylmercapto)-hydantoin, alkaline hydrolysis of which leads to an alkali salt of methionine. Neutralization with an acid such as carbon dioxide or sulphuric acid gives racemic methionine, several hundred thousand tonnes of which are produced annually.
A conventional method is based on the use of circulated alkaline potassium salts for hydrolysis of 5-(2-methylmercapto)-hydantoin. Unwanted by-products lead to neutral potassium salts, which are then no longer available to the alkaline hydrolysis reaction. These must be removed from the potassium-containing circulating solution. The associated potassium losses must be compensated by using KOH. Another side reaction leads to the formation of 4-methylmercapto-2-hydroxybutanoic acid and therefore to a loss of yield. 4-Methylmercapto-2-hydroxybutanoic acid is also present as neutral potassium salt in the bottoms product of reactive distillation and therefore disturbs the circulation of alkaline potassium. This by-product is therefore unsuitable for supporting the hydrolysis of 5-(2-methylmercapto)-hydantoin and must be removed from the potassium circuit, which is associated with further losses of raw materials.
It is known from the background art that methionylmethionine (also called methionine dipeptide) is a by-product in methionine production by hydrolysis of hydantoin (e.g. EP 2 133 329 A2, EP 0839 804 B1) and forms in the following way (formula I).

A two-stage hydantoin hydrolysis for reducing the formation of methionine dipeptide is described in EP 2 133 329 A2. In this, the first stage takes place in a flow tube with gas outlet and the second stage in a stirred reactor, with total residence times of 20-60 minutes. This gives a product distribution of 91 mol. % methionine and 9 mol. % methionine dipeptide. The high proportion of methionine dipeptide is very unfavourable and requires further process steps to reduce losses of yield. Thus, EP 2 186 797 A1 describes expensive heat treatment at 150-200° C. of the concentrates of the mother liquors from methionine crystallization, to cleave methionine dipeptide hydrolytically back to methionine. For this, after crystallization, which takes place at 20° C., the mother liquor must be heated again to the required high temperatures, which is energetically unfavourable.
JP 2006-206534 A describes the depletion of NH3 from a process solution produced in an additional process step by hydrolysis of hydantoin at normal pressure and therefore at about 100° C. by means of a plate column. This is a normal distillation of NH3, having no effect on the hydrolysis reaction. A drawback of this procedure is that the NH3 thus removed is no longer available for hydantoin synthesis. Moreover, the solution thus treated is heavily diluted, as stripping steam at a pressure of 5 bar (excess) (corresponding to 158° C.) is used, which is disadvantageous for further processing.
EP-A-1710232 and EP-A-1256571 describe a method of production of D,L-methionine from 5-(2-methylmercapto)-hydantoin, wherein the process stages 5-(2-methylmercaptoethyl)-hydantoin formation and methioninate formation (alkaline hydrolysis) can be operated continuously and integrated successively in a fully continuous process. How a column for hydrolysis of Met-hydantoin should be designed technically to minimize by-product formation has not yet been described in the background art. Furthermore, there is no description of how the NH3 circuit between hydantoin formation and hydantoin hydrolysis is to be operated loss-free at minimized energy expenditure. The hydrolysis process can be carried out in a steam-heated column, wherein the 5-(2-methylmercaptoethyl)-hydantoin solution is advantageously fed continuously to the top of the column at a rate such that the hydrolysis product, potassium methioninate solution, can accordingly be withdrawn at the bottom of the column after quantitative hydrolysis. The mother liquor can be reused after separating the methionine solids. The gaseous constituents (steam, ammonia and carbon dioxide) can be discharged at the top of the column and can be used for restoration of the aqueous ammonia/carbon dioxide solution for production of 5-(2-methylmercaptoethyl)-hydantoin. It is described as being especially advantageous, for avoiding by-products, to carry out the hydrolysis right at the start in the presence of alkali and carbon dioxide, i.e. in particular there is a mixture of alkali compounds, in particular alkali hydrogen carbonate, alkali carbonate, alkali hydroxide. To achieve complete conversion of the valuable starting material MMP to hydantoin, a molar ratio of 1.005-1.02 mol/mol HCN/MMP is employed in the method described. As a result of hydrolysis of unreacted hydrogen cyanide, in the column there is production of NH3 by the following reaction (formula II):HCN+2H2O→NH3+HCOOH   II
The resultant formic acid is located as non-volatile potassium formate at the bottom of the reaction column; it thus displaces the required alkaline carbonates and therefore disturbs the alkaline potassium circuit.